linux-kernel - Re: Why do we set _PAGE

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Message-ID: <CA+55aFwsPs12_57YEBHdb4ti1BXSuDX_RPSf6S4JSRLGK_2X7Q@mail.gmail.com>
Date:	Tue, 22 Apr 2014 19:48:54 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Why do we set _PAGE_DIRTY for page tables?

On Tue, Apr 22, 2014 at 4:35 PM, H. Peter Anvin <hpa@...or.com> wrote:
> I just noticed this:
>
> #define _PAGE_TABLE     (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER |       \
>                          _PAGE_ACCESSED | _PAGE_DIRTY)
> #define _KERNPG_TABLE   (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |   \
>                          _PAGE_DIRTY)
>
> Is there a reason we set _PAGE_DIRTY for page tables?  It has no
> function, but doesn't do any harm either (the dirty bit is ignored for
> page tables)... it just looks funny to me.

I think it just got copied, and at least the A bit does matter even in
page tables (well, it gets updated, I don't know how much that
"matters"). So the fact that D is ignored is actually the odd man out.

               Linus
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